This invention relates to batteries and more particularly to alkaline nickel-zinc batteries.
Alkaline nickel-zinc batteries are potentially attractive high rate, high energy density, cost effective power sources. However, their useful life has been limited by a number of factors. These include premature shorting of the battery cells by zinc penetration and the gradual loss in electrical capacity due to corrosion, slumping, and densification of the zinc anodes (shape change) during charge/discharge cycling of the batteries. Secondary problems associated with nickel-zinc batteries are hydrogen gassing, with the resulting loss of water in the batteries, and abuse to the batteries due to improper charging controls.
Methods used to prevent or hinder zinc dentrite penetration of the separator include using thick diffusion-type separators or having a layer of low hydrogen overpotential material included within the separator wrap. A serious disadvantage of solely using the thicker separators is that they usually induce the formation of large concentration gradients, particularly when employed as several layers. The large concentration gradients produce electroosmotic pumping effects which promote washing and shape change of the zinc anodes with resulting loss of electrical capacity by the cell.
U.S. Pat. No. 4,192,908 discloses a separator in which a 50 to 1000 A layer of nickel is coated onto a 1 mil microporous polyolefin membrane which is then sandwiched between two uncoated 1 mil microporous polyolefin layers. This separator was effective in preventing zinc dendrite penetration without creating serious concentration gradient problems. Thus, shape change of the zinc anodes was also minimized. However, after a substantial number of cycles the nickel coating would shed and the cell would fail. It was necessary to prevent this shedding of the nickel layer in order to substantially increase the life of the cell.
Additives to the zinc anode and methods of charging the cell have also been used to minimize the shape-change of zinc anodes. U.S. Pat. No. 3,623,911 issued to Harry G. Oswin discloses the use of metals from Groups III and IV-B (In, Tl, Pb and Ga) to zinc anodes to prevent passivation. U.S. Pat. No. 4,084,047 issued to Albert Himy and Otto C. Wagner discloses the addition of binary mixtures of Tl, Pb, Cd, In, Sn and Ga to zinc anodes to prevent hydrogen gassing, shape change of the anode, and to reduce the self discharge of the cells. U.S. Pat. No. 3,556,849 issued to Harry G. Oswin and Keith F. Blurton discloses a method of pulse changing alkaline zinc cells with the resulting reduction in shape change of the zinc anode.
Although all the cited state-of-the-art innovations have improved the performance and cycle life of nickel-zinc cells up to a maximum of 250 charge/discharge cycles, it is a practical necessity to double the useful life of the nickel-zinc battery for most required applications. It will be demonstrated that this can be accomplished by the employment of the innovations of the present invention.